PARKLAND, Fla. — At least 17 people were killed Wednesday in a high school shooting in Parkland, Florida, Broward County Sheriff Scott Israel said.

The suspect, 19-year-old former student Nikolas Cruz, is in custody, the sheriff said. Cruz was expelled for unspecified disciplinary reasons, and police are investigating his digital profile, Israel said. So far, what they've found is "very, very disturbing," he said.

"It's a horrific, horrific day," Israel said. "Just pray for this city. Pray for this school, the parents, the folks that lost their lives."

Law enforcement responded to reports of a shooting at Marjory Stoneman Douglas High School shortly before 3 p.m. and encountered a chaotic scene. Investigators believe the suspect pulled the fire alarm to draw people out of classrooms, a law enforcement source told CNN. Students said a fire drill earlier in the day added to the confusion, causing some to think it was a false alarm.

Seventeen people, including the suspect, were sent to area hospitals, said Dr. Evan Boyar of Broward Health. The suspect was treated and released to police.

The victims included students and adults, the sheriff said. Twelve were killed inside the building and two died outside, he said. One died in the street and two died at the hospital, Israel said.

The shooting sent students and staff streaming out of the school in a panic. Immediately after the shooting, aerial footage from CNN affiliate WSVN showed people lying on the ground outside the school, being treated for injuries and moved to ambulances.

Freshman Kayden Hanafi said he heard two gunshots and saw people running out of another building on campus. As he and his classmates went into lockdown in a classroom, many thought the noise might have been firecrackers.

"It's really a blessing to still be alive," he said.

Nicole Baltzer, 18, said she was in trigonometry class about 10 minutes before the end of the school day when the fire alarm went off. As students evacuated, she heard six gunshots and everyone started running back inside the school, Baltzer told CNN's Sara Ganim.

"I heard so many gunshots, at least like six. They were very close," Baltzer said.

A police officer told her to close her eyes as she walked past a classroom with broken glass, telling her "there's nothing good to see in there," she said.

A freshman named Aidan posted a photo on social media from inside his math class while on lockdown. Since then, he has left the building.

"We have been liberated. God bless, America," Aidan tweeted after being evacuated from the building. "Love each other. You may never know when it may be the last day you meet someone."

There had been a fire drill at the school earlier in the day, leading some to believe at first that the afternoon incident was another drill, a student told CNN affiliate WSVN. "Everyone just started freaking out."

"But then word started going around that it was shots and not just, like, something else, everyone just started running towards the canal," the student said.

Parkland, with a population of 31,000 people in 2016, was named Florida's safest city last year, according to an analysis by the Washington-based National Council for Home Safety and Security, a home security industry trade association. The south Florida city had seven reported violent crimes and 186 property crimes the previous year, according to the analysis.

Stoneman Douglas High School had an enrollment of about 3,100 students in the 2015-16 year, according to the National Center for Education Statistics.

As news of the shooting spread, desperate parents gathered near the property searching for their children.

Lissette Rozenblat told CNN that her daughter safely evacuated the school and took shelter at a nearby Walmart.

"She was very nervous, she said that she could hear the person who was shot crying out for help, and was just a nervous wreck," Rozenblat said.

Agents from the Miami office of the Bureau of Alcohol, Tobacco, Firearms and Explosives are responding to the scene, a spokesman said.

Officials responded to reports of a shooting at Marjory Stoneman Douglas High School in Parkland where police said 14 people were injured.

Aerial footage from WSVN showed a number of people lying on the ground outside the school, being treated for injuries and moved to ambulances. As students streamed away from the school, some ran while others walked quickly in orderly lines with their hands on or above their heads.

Nicole Baltzer, 18, said she was in trigonometry class about 10 minutes before the end of the school day when the fire alarm went off. As students evacuated, she heard six gunshots and everyone started running back inside the school, Baltzer told CNN's Sara Ganim.

"I heard so many gunshots, at least like six. They were very close," Baltzer said.

Baltzer said she is still on lockdown in a classroom on the second floor. Her stepsister is also on lockdown in a different classroom, she said.

"We all thought it was a fire drill because we had one previously today," he said. "And we thought it was, so no one was that nervous, but then word started going around that it was shots and not just, like, something else, everyone just started running towards the canal."

Lissette Rozenblat told CNN that her daughter safely evacuated the school and was taking shelter at a nearby Walmart.

"She was very nervous, she said that she could hear the person who was shot crying out for help, and was just a nervous wreck," Rozenblat said.

Dozens of law enforcement vehicles are on site, including one tank, along with a team of heavily armed law enforcement officers. Agents from the Miami office of the Bureau of Alcohol, Tobacco, Firearms and Explosives are responding to the scene, a spokesman said.